This invention relates to a process for treating wastewaters with a powdered adsorbent and one or more fixed media filters, also known as biological packed towers or trickling filters.
Fixed media filters have been commonly used for treating relatively weak wastewaters, especially in the aerobic embodiment known as the trickling filter. Typically, trickling filters are comprised of towers filled with crushed rock, field stones, wooden slats or plastic media which not only provide a large surface area for growth of biota, but whose interstitial spaces have dimensions adequate for flow of liquid downward and air upward or downward without large pressure drops or clogging with solids. Preclarification of wastes which contain solids is generally considered necessary to prevent clogging of the media void spaces. Media, such as rock, are usually 2 to 4 inches in diameter, with the larger material size prevalent in high rate filters. Recently developed plastic media provided even greater void space dimensions. Recirculation of wastewaters through the media is often provided to better maintain the biota viability and enhance the treatment level. Suggested flow diagrams can be found in Sewage Treatment Plant Design, WPCF Manual of Practice No. 8 (1977), Water Pollution Control Federation, pages 283-309. While the air flow in most trickling filter installations results from natural convection in the bed, forced aeration using blowers has also been used. See for example Dekema, U.S. Pat. No. 2,308,866 and Burkhead, U.S. Pat. No. 3,966,599.
In practice, trickling filters have been unable to attain the same removal of 5-day biochemical oxygen demand (BOD5) as for instance, activated sludge systems. Their use has usually been limited to applications where effluent BOD5 of 20 to 40 mg/l is acceptable, or where partial treatment is applicable. Substantial nitrification has rarely been achieved in existing trickling filter installations. When it has occurred, denitrification in the final clarifier has often caused flotation of suspended solids and carryover into the treated wastewater effluent.
Ideally, biological solids which form on the media surfaces will naturally slough from the surfaces at the same rate as formed, to maintain a large viable population of organisms for removing wastewater constituents. In practice, however, sloughing often occurs intermittently, usually at times of overloading or when toxic materials are introduced into the wastewater. At such times, when the need for a high degree of treatment is greatest, the trickling filter loses much of its biological solids, and much reduced removals of BOD5 and suspended solids will occur and persist for long periods of time, often several weeks. Nitrification takes even longer to become re-established than carbonaceous BOD5 removal following such an upset.
The effluent quality of most trickling filter installations deteriorates under winter climatic conditions. The winter BOD5 removal is often 20-25 percent less than summertime removal.
Recent changes in some effluent standards have required removal of BOD5 to levels of 5-15 mg/l or lower, in addition to substantial nitrogen removal. Trickling filters per se have generally been unable to meet these standards; alternate, more expensive treatment methods have been proposed and used.
Packed columns of adsorbent such as granular activated carbon have been used for removing adsorbable components from waste streams, where only trace quantities of suspended solids are present in the waste. Even small amounts of solids will eventually clog the interstitial spaces and cause excessive headloss through beds of even the largest available carbon particles of 2 to 3 mm or larger diameter. Biological growths in or on the bed also severely restrict fluid flow and have prevented the use of carbon columns in such cases.
Upflow expanded beds have been used to eliminate plugging problems. The upward flow of liquid or liquid and air causes the carbon particles to become suspended in the mixture. Granular carbon of 0.4 to 1.7 mm particle diameter is typically used because of performance and cost advantages.
Using adsorbents such as activated carbon in mixed vessels is also known. For example, Timpe et al., U.S. Pat. No. 3,763,040 shows continuous water treatment in a plurality of tanks, where the activated carbon is transported countercurrent to the water flow.
Activated carbon is claimed to act as a catalyst to chemical oxidation under certain conditions. For example, Urbain et al., U.S. Pat. No. 2,086,753 discloses the non-biological oxidation of wastewater contaminants by sulfonated active carbon in the presence of hydrogen ion, iron and an oxygen containing gas. Also, Hoke, U.S. Pat. No. 3,817,862 shows oxidation of contaminants in the presence of activated carbon, independent of biological action.
More recently, combining biological oxidation with powdered activated carbon adsorption in the same mixed aerated vessel has become known. See Blecharczyk, U.S. Pat. No. 3,803,029 and Hutton et al., U.S. Pat. No. 3,904,518.
Regeneration of powdered activated carbon by aerobic or anaerobic biological treatment is shown in Blecharczyk, U.S. Pat. No. 3,803,029. Regeneration by wet oxidation is disclosed in Schoeffel et al., U.S. Pat. No. 3,442,798 and the incorporation of wet oxidation regeneration into an aerobic biological-physical treatment system is shown in Pradt et al., U.S. Pat. No. 3,977,966. In the later disclosure, the toxic waste is wet oxidized together with spent adsorbent to degrade the toxic substances and regenerate the carbon adsorbent prior to biologically treating the wastewater in combination with adsorbent.